(Thanks to Mark Lavin for significant contributions to this post.)
In a previous post, we introduced using Celery to schedule tasks.
In this post, we address things you might need to consider when planning how to deploy Celery in production.
At Caktus, we've made use of Celery in a number of projects ranging from simple tasks to send emails or create image thumbnails out of band to complex workflows to catalog and process large (10+ Gb) files for encryption and remote archival and retrieval. Celery has a number of advanced features (task chains, task routing, auto-scaling) to fit most task workflow needs.
A simple Celery stack would contain a single queue and a single worker which processes all of the tasks as well as schedules any periodic tasks. Running the worker would be done with
python manage.py celery worker -B
This is assuming using the django-celery integration, but there are plenty of docs on running the worker (locally as well as daemonized). We typically use supervisord, for which there is an example configuration, but init.d, upstart, runit, or god are all viable alternatives.
The -B option runs the scheduler for any periodic tasks. It can also be run as its own process. See starting-the-scheduler.
We use RabbitMQ as the broker, and in this simple stack we would store the results in our Django database or simply ignore all of the results.
In a large setup we would make a few changes. Here we would use multiple queues so that we can prioritize tasks, and for each queue, we would have a dedicated worker running with the appropriate level of concurrency. The docs have more information on task routing.
The beat process would also be broken out into its own process.
# Default queue python manage.py celery worker -Q celery # High priority queue. 10 workers python manage.py celery worker -Q high -c 10 # Low priority queue. 2 workers python manage.py celery worker -Q low -c 2 # Beat process python manage.py celery beat
Note that high and low are just names for our queues, and don't have any implicit meaning to Celery. We allow the high queue to use more resources by giving it a higher concurrency setting.
Again, supervisor would manage the daemonization and group the processes so that they can all be restarted together. RabbitMQ is still the broker of choice. With the additional task throughput, the task results would be stored in something with high write speed: Memcached or Redis. If needed, these worker processes can be moved to separate servers, but they would have a shared broker and results store.
Creating additional workers isn't free. The default concurrency uses a new process for each worker and creates a worker per CPU. Pushing the concurrency far above the number of CPUs can quickly pin the memory and CPU resources on the server.
For I/O heavy tasks, you can dedicate workers using either the gevent or eventlet pools rather than new processes. These can have a lower memory footprint with greater concurrency but are both based on greenlets and cooperative multi-tasking. If there is a library which is not properly patched or greenlet safe, it can block all tasks.
There are some notes on using eventlet, though we have primarily used gevent. Not all of the features are available on all of the pools (time limits, auto-scaling, built-in rate limiting). Previously gevent seemed to be the better supported secondary pool, but eventlet seems to have closed that gap or surpassed it.
The process and gevent pools can also auto-scale. It is less relevant for the gevent pool since the greenlets are much lighter weight. As noted in the docs, you can implement your own subclass of the Autoscaler to adjust how/when workers are added or removed from the pool.
Task state and coordination is a complex problem. There are no magic solutions whether you are using Celery or your own task framework. The Celery docs have some good best practices which have served us well.
Tasks must assert the state they expect when they are picked up by the worker. You won't know how much time has passed since the original task was queued and when it executes. Another similar task might have already carried out the operation if there is a backlog.
We make use of a shared cache (Memcache/Redis) to implement task locks or rate limits. This is typically done via a decorator on the task. One example is given in the docs though it is not written as a decorator.
When getting started with Celery you must make two main choices:
- Result store
The broker manages pending tasks, while the result store stores the results of completed tasks.
There is a comparison of the various brokers in the docs.
As previously noted, we use RabbitMQ almost exclusively, though we have used Redis successfully and experimented with SQS. We prefer RabbitMQ because Celery's message passing style and much of the terminology was written with AMQP in mind. There are no caveats with RabbitMQ like there are with Redis, SQS, or the other brokers which have to emulate AMQP features.
The major caveat with both Redis and SQS is the lack of built-in late acknowledgment, which requires a visibility timeout setting. This can be important when you have long running tasks. See acks-late-vs-retry.
To configure the broker, use BROKER_URL.
For the result store, you will need some kind of database. A SQL database can work fine, but using a key-value store can help take the load off of the database, as well as provide easier expiration of old results which are no longer needed. Many people choose to use Redis because it makes a great result store, a great cache server and a solid broker. AMQP backends like RabbitMQ are terrible result stores and should never be used for that, even though Celery supports it.
Results that are not needed should be ignored, using CELERY_IGNORE_RESULT or Task.ignore_result.
To configure the result store, use CELERY_RESULT_BACKEND.
RabbitMQ in production
When using RabbitMQ in production, one thing you'll want to consider is memory usage.
With its default settings, RabbitMQ will use up to 40% of the system memory before it begins to throttle, and even then can use much more memory. If RabbitMQ is sharing the system with other services, or you are running multiple RabbitMQ instances, you'll want to change those settings. Read the linked page for details.
Transactions and Django
You should be aware that Django's default handling of transactions can be different depending on whether your code is running in a web request or not. Furthermore, Django's transaction handling changed significantly between versions 1.5 and 1.6. There's not room here to go into detail, but you should review the documentation of transaction handling in your version of Django, and consider carefully how it might affect your tasks.
There are multiple tools available for keeping track of your queues and tasks. I suggest you try some and see which work best for you.
When going to production with your site that uses Celery, there are a number of decisions to be made that could be glossed over during development. In this post, we've tried to review some of the decisions that need to be thought about, and some factors that should be considered.